A remarkable new glazing system has been available in Europe for several years and is now being brought to the North American market. The GlassX window is an insulated glass assembly that incorporates a phase-change material (PCM) between two of the glass panes in the window. At lower temperatures, the PCM is a translucent solid. But, as it heats, the PCM melts and becomes transparent. This lets the window itself absorb heat from solar energy during the day, and then releases the energy again later on, as the material cools again.

The combination of good insulating windows along with heat storage makes these windows very useful for passively designed buildings. PCMs are excellent heat storage materials, and the GlassX windows are able to store as much heat as a 9" thick concrete wall. Even in its solid, translucent state, the GlassX windows allow more than 25% of the exterior light through, so that daylighting is not entirely lost. The windows also incorporate a diffuser that reflects high angle light from the sun in summertime, while allowing low angle light in the winter to pass through more directly.